Steam methane reforming of a hydrocarbon containing feed is practiced to produce hydrogen for such refinery uses as hydrotreating and hydrocracking. As a result, steam methane reformers are operated in connection with fuel refining facilities. A variety of off gas streams are produced in refineries from processes such as fluidic catalytic cracking, coking, catalytic reforming, hydrocracking and etc. Generally, all these streams are used for fuel in process heaters and in steam generators for making steam.
However, as may be appreciated, the combustion of fuels produces carbon dioxide. The production of carbon dioxide is seen as a contributor to greenhouse gases and potential climate change. Consequently, there has been a worldwide effort to reduce greenhouse gases that includes reducing the production of carbon dioxide in the atmosphere. As indicated in Philips “CO2 Management in Refineries”, Foster Wheeler Energy Limited (2002), process heaters produce roughly half of the carbon dioxide that is emitted from refinery operations to the atmosphere as greenhouse gases. As indicated in this paper, the gasification of refinery off gases could be used to produce fuels and hydrogen that would allow for a relatively easy capture of half the carbon dioxide emissions in the refinery. The captured carbon dioxide could be used in enhanced oil recovery processes.
In Simmonds et al., “A study of very large scale post combustion carbon dioxide capture at a refining and petrochemical complex”, it has been proposed to gather flue gas produced in the refinery and then capture the carbon dioxide in a carbon dioxide capture plant to sequester the carbon dioxide for later use. It is proposed in this reference to capture the carbon dioxide by a duct network of approximately 2 kilometers in length and having a maximum cross-sectional area of 9 square meters. It is envisioned in this paper that the carbon dioxide be captured through the use of amine scrubbing units.
Another source of carbon dioxide emissions is the steam methane reformer itself. A steam methane reformer produces a synthesis gas in which hydrogen can be separated or used in down stream chemical processes. Such down stream processes include the production of methanol and gas to liquid processes for synthetic fuels by means of the Fischer-Tropsch process. In any case, syntheses gas is generated within a steam methane reformer by introducing a hydrocarbon containing feed, typically natural gas, into reformer tubes located within a radiant section of the steam methane reformer. The reformer tubes contain a catalyst to promote the steam methane reforming reactions in which steam is reacted with the hydrocarbons to produce hydrogen, carbon monoxide, water and carbon dioxide. The resulting reformed stream is further reacted within one or more water-gas shift reactors in which the hydrogen content is increased by reacting carbon monoxide with steam. This results in additional carbon dioxide being formed. Typically, the resulting shifted stream is cooled and then introduced into a pressure swing adsorption unit in which the hydrogen is separated. Such separation of the hydrogen results in a tail gas stream being produced that contains hydrogen, methane, carbon monoxide, and carbon dioxide. The tail gas stream is used in part in firing the burners of the reformer and also in process steam heaters for raising steam that is used in the steam methane reforming reaction. All of this combustion adds to the carbon dioxide emissions of the facility.
In U.S. patent application Ser. No. 2007/0232706 A1, it has been proposed to capture the carbon dioxide in the shifted stream by first separating carbon dioxide in a vacuum pressure swing adsorption unit in which the adsorbent adsorbs the carbon dioxide to produce a hydrogen-rich stream that is introduced into the pressure swing adsorption unit. During desorption of the adsorbent, a carbon dioxide-rich stream is produced that is further processed by being compressed, cooled, dried, purified and then subjected to a sub-ambient temperature distillation process within a distillation column. Resulting bottoms liquid is vaporized within a main heat exchanger that is used in connection with the sub-ambient temperature distillation process to produce a carbon dioxide-rich stream that can be further compressed and used for down stream processes, for example, enhanced oil recovery or sequestered. Carbon dioxide lean vapor that is produced as a result of the distillation can be warmed and recycled back to the pressure swing adsorption unit for further processing along with the incoming synthesis gas feed.
Refinery off gases are a hydrocarbon containing stream that could potentially be reformed in a steam methane reformer. However, such streams often have a high olefin content that will deactivate the catalyst within the steam methane reformer. In U.S. Pat. No. 7,037,485, it is disclosed that such a refinery off gas stream can be compressed and treated within a guard bed to remove metal and sulfur species and then either alone or combined with natural gas introduced into a reactor to catalytically react the hydrogen with the hydrocarbons and any sulfur compounds to form saturated hydrocarbons and hydrogen sulfide. If the incoming stream does not contain sufficient hydrogen, hydrogen can be recirculated from the pressure swing adsorption unit. Alternatively, an oxygen stream can also be added to react with the hydrocarbons, the hydrogen and sulfur compounds contained within the feed so that additional hydrogen and carbon monoxide are produced. The resultant stream can then be passed into sulfur removal beds such as a zinc oxide bed to remove hydrogen sulfide and the resulting stream can then be combined with steam and safely reacted within the steam methane reformer.
As will be discussed, the present invention provides a method of reducing carbon dioxide emissions within a refinery that, among other advantages, does not require complex piping networks to gather flue gas and does not produce a carbon dioxide-rich tail gas as dioxide that is separated in accordance with the present invention is thereby sequestered from the fuel and is available for other processes such as enhanced oil recovery.